This invention relates to organic based spintronic devices, and electronic devices comprising them, such as spin valves (such as those shown in FIGS. 1 and 3), spin tunnel junctions, spin transistors and spin light-emitting devices that use the arrangements of the present inventions.
1. Innovation and Concepts
New polymer-, organic- and molecular-based electronic devices in which the electron spin degree of freedom controls the electric current to enhance device performance provide a key basis for a broad range of technologies. Polymer-, organic-, and molecular-based spintronic devices have enhanced functionality, ease of manufacture, are less costly than inorganic ones. The long spin coherence times due to the weak spin-orbit interaction of carbon and other low atomic number atoms that comprise organic materials make them ideal for exploiting the concepts of spin quantum devices. The hopping mechanism of charge transport that dominates in semiconducting polymers (vs. band transport in crystalline inorganic semiconductors) enhances spin-magneto sensitivity and reduces the expected power loss.
Introduction
In the past decade there has been extensive progress in inorganic multilayer based spin valves.1 Giant magnetic resistance and spin valves based on this effect recently enabled 100% per year growth of the areal density in the hard drive disk industry.2 Recent extensions to magnetic semiconductor-based spin valves3 and related spin-LEDs4 have been shown to be promising embodiments of spintronics; however, there are substantial problems due to interfaces as well as low Curie temperatures (Tc) for present magnetic semiconductors.5 
There is growing interest in replacing inorganic electronic materials with inexpensive, easier-to-fabricate polymers akin to the interest in using conducting polymers in a myriad of electronic applications.6 Many polymer/molecular/organic materials can be dissolved in solution and spun into thin films or readily evaporated onto many substrates. This is anticipated to lead to cost efficiency and ease of manufacture in devices, especially for large area use and flexible substrates. Also, electronic polymers are known to be radiation hard. Another advantage of molecular/organic materials is the richness of chemistry which enables the synthesis of materials with very specific properties. Recently it was demonstrated7 that organic molecules can be used to create nano-sized heterojunctions and as building blocks for a molecular computer.8 
Today, selected polymers readily conduct electric charge.9 The room-temperature conductivity "sgr"RT of conjugated polymers, such as polyacetylene, polyaniline, and polypyrrole, can be controlled over 15 decades by doping and structural order and attain a value that is only an order of magnitude below the record conductivity of superpure Cu ("sgr"RT=6xc3x97105 S/cm).
Since the report in 1990 of electroluminescence10 in the light emitting polymer (LEP) PPV, there have been many advances in polymer light-emitting diodes. Polymer/molecule based LEDs in all wavelengths of light11 (from ir to uv) and with a wide variety of output parameters are known12 and commercialization of polymer LEDs has begun.13 
Molecule-based magnetism started with the discovery14 in the mid 1980""s of ferromagnetism at 4.8 K in the linear chain of electron transfer salt [FeCp*2][TCNE] (Cp*=pentamethylcyclopendienide; TCNE=tetracyanoethylene) by Miller and Epstein. Today there is xe2x80x9cpolymericxe2x80x9d material such as V[TCNE]x (xxcx9c2) that is room temperature magnet and which is processable at room temperature by using conventional organic chemistry, common solvents, and low temperature chemical vapor deposition (CVD).15 V[TCNE]x is a xe2x80x98softxe2x80x99 magnet with magnetic ordering temperature Tc=400 K, a small coercive field Hc=4.5 Oe (varying by more that one order of magnitude with details of preparation/processsing) at 300 K, and semiconducting conductivity "sgr"RTxcx9c10xe2x88x925 S/sm. Other examples of polymer/molcule/organgic based magnets include Prussian Blue(s). Examples of Prussian Blue(s) structure-containing compounds include V[Cr(CN)6]xxc2x7YH2O, where x is between 0.5 and 1.5; preferably 0.8 to 1.2, and Y is between 0 and 4; preferably 1.5 and 2.8.
Approach/innovative Concepts
We propose the concept of spintronics in polymer devices as all the mandatory elements can be achieved with polymers. A key argument for exploiting polymer spintronics is the very long spin coherence time (xcfx84s) in polymers. Analysis of EPR data yields 10xe2x88x927 s for poorly conducting polymers16 and microseconds for well-conducting samples.17 Also, xcfx84s is 10xe2x88x928 s for V[TCNE]x18, which is longer than that of conventional inorganic semiconductors xcx9c10xe2x88x929 s. Polymers enable us to overcome difficulties that inorganic spintronics faces such as poor spin injection through the interfaces, low Curie temperature, and low sensitivity. Preliminary measurements yield low barriers to charge injection between magnetic and conducting polymers. Table 1 is a comparative summary of semiconductor and polymer-based magnet parameters.
The schematic layout of a vertical polymer-based magneto-spin gate (spin-valve) is shown in FIG. 1. The resistance of the sandwich structure (Hard Magnetxe2x80x94Conductorxe2x80x94Soft Magnet) strongly depends on the relative orientation of the magnetization of hard and soft magnets. The hard magnet aligns spins of electrons injected from the metal contact. The transit time for electrons across the central conductor is shorter than the spin coherence time; therefore, if the magnetization of the soft magnet is aligned with that of the hard magnet, electrons easily continue their path to reach another electrode. For opposite magnetization of the soft magnet, transit of the polarized electrons is forbidden unless the spins reorient. The conducting layer in the center serves as a spacer to separate soft and hard magnets, thus, enabling the soft magnet to be tunable by an external magnetic field.
Estimates of the parameters and characteristics of a polymer spin-valve vary depending on mechanism of charge transport. In highly conducting polymers ("sgr"RTxe2x89xa6100 S/cm) the room-temperature conductivity is provided by metallic band-like motion, with charge hopping over nearest neighbor states.19 Then the hopping rate xcfx89h is given by the typical phonon frequency 1012 sxe2x88x921 and the length of hopping Lhop is the localization radius "xgr", which is given by scale of inhomogenieties, typically 2 nm. Taking the spin coherence time xcfx84s as 10xe2x88x927 s we find that the electron makes Nhxcx9c105 hops before the electron loses its spin orientation. Hence, the spin coherence length can be estimated as Lcoherence≈Lhopxc3x97Nhxcx9c1 xcexcm.
If the poorly conducting polymers ("sgr"RTxe2x89xa610xe2x88x921 S/cm) are used, the Mott variable range hopping (VRH) mechanism of transport dominates. In this case the hopping rate is essentially decreased, but simultaneously the length of hopping increases; therefore, on the whole the coherence length Lcoherence remains at the micron scale at least for room temperature. It is important to have Lcoherence large, as this parameter controls the effectiveness of the device and the spin coherence length also limits the allowed thickness of conducting layer LC less than Lcoherence. For a hard magnet, a metallic ferromagnetic or ferrimagnetic or similar film may be used. In soft polymeric magnets, such as V[TCNE]x, charge motion is by hopping and the hopping distance is determined by the disorder. We estimate the hopping length as 10 nm. The thickness of the soft magnet LS should be at least a few hopping distances, i.e., LS greater than  greater than 10 nm. Other polymer, organic or molecule based magnets may be used, including those with spins in organic groups only, those with spins on metals and on organics, and those with spins only on inorganic groups connect through spinless organic groups. It is noted that nearly fully spin polarized transport may be achieved by use of ferrimagnetic order between to groups of different ionization energy. For example, for V[TCNE]x, the spin {fraction (3/2)} of the V is opposite to the spin xc2xd of the TCNE. This leads to the charge carriers (coming from the TCNE) being polarized (up to 100% polarized if x=2). To read a magnetic memory, the external field for the soft magnet should be a few Gauss. V[TCNE]x with coercive force 4.5 Oe is close to this requirement.
We estimate the potential sensitivity of polymer spin-valves as follows. Through changing the magnetic polarization of the soft magnet from parallel to antiparallel with respect to the hard magnet, the activation energy for electrons in the soft magnet area increases. Therefore, the change of conductance xcex94G with polarization is given by xcex94G/G=(1xe2x88x92exp[xe2x88x92xcex94E/(kBT)]), where G is the conductance of the device. We assume that G is determined by the soft magnetic layer which is the most resistive part of sandwich. xcex94E is the activation energy, which is given by the internal field Hi of the soft magnet: xcex94E=gxcexcBHi. According to mean field theory, Hi is temperature dependent20, and, therefore, xcex94E=kBTc(1xe2x88x92T/Tc)xc2xd, where Tc is the temperature of magnetic ordering. For Tc=400 K and T=300 K we obtain xcex94G/G=40%. The above estimate gives an approximate scale for sensitivity, neglecting the thickness of soft magnetic film and the energy disorder by considering it as a monolayer with one hopping energy. FIG. 2 presents promising initial bulk magnetoresistance for CVD-prepared V[TCNE]x film, recording an xcx9c0.3% change in resistance at room temperature for an applied field of 6 kOe.
For magnetic memory applications, e.g., read heads and magnetic random access memories, the current in plane (CIP) spin-valve construction, FIG. 3, is common. Here we replace the hard magnet layer with a polymer, molecule or organic based soft magnet polarized by the hard magnet. The central conducting spacer will be thin so that the total conductance principally is determined by transport through the soft magnet area. As "sgr"RT for V[TCNE]x is xcx9c10xe2x88x925 S/cm, less conducting polymers with VRH conductivity will be used. Again as "sgr" in polymeric magnets is dominated by the activated hopping we expect the spin-magneto sensitivity to be very high and, perhaps, it can approach its maximum, xcex94G/G=50%. The small absolute G guarantees low power loss of device. For the polymer, organic or molecular based spin valve the nonmagnetic conductor may be an inorganic conductor or semiconductor such as copper or silicon, or an organic, molecular or polymer based conductor such as doped polythiophene or doped polyaniline, or semiconductor, such as undoped polythiophene, polyaniline, or tetracene.
Novel Polymer Spin LED
The schematic construction of an all-polymer Spin-LED, FIG. 4, has all central working elements made from polymers. Typically transparent indium-tin oxide (ITO) with high work function is used as a hole-injecting electrode. Al can be chosen to serve as the cathode as its work function is small. The electron affinity of V[TCNE]x is expected to be close to the Fermi level of Al. Spins of electrons injected from the Al contact are aligned by the strong internal field of the polymer magnet. The orientation of the internal magnetic field follows the weak applied external field. The choice of light-emitting polymer (LEP) is determined by the desired wavelength of light and need for chirality and spin orbit coupling to achieve circularly polarized emission. Examples of spin LED emitters include rare earth metal containing polymers and metal complexes. For examples, Er containing molecules (for ir emission at 1.5 microns, important for fiber optic communications), green emitting Ir and Pt containing complexes for
Addition of rare earth ions with their large spin-orbit coupling will increase the circularly polarized emission and quantum yield of electroluminescence. Chiral groups and dopants as as phosphorescent dyes may also be used for achieving circularly polarized emission. The prevailing spins in LEP can be detected by increase in one of the circularly polarized components similar to magnetic circular dichroism.22 Thus the weak external magnetic field leveraged by the soft magnet and transferred by electrons into the optically active part finally controls dichroism in the device. While Tc greater than 300 K for the magnets is routinely achievable, the interfaces between magnetic polymers and other materials is important. Coating of the films with, for example, inert polymers enhances the longevity of films and devices by making them more stable to ambient conditions. This includes greater chemical stability to air, water, and solvents, as well as abrasion resistance.
Improved performance of next generation systems requires enhanced magnetic polymers. Other organic, polymer, and molecule based thin-film magnets are based on other acceptors instead of TCNE. For example, V[TCNQ]x (TCNQ=7,7,8,8-tetracyano-p-quinodimethane) forms thin film magnets and has different magnetic behaviors. Likewise, improved materials are expected from optimization of the vanadium source for the V[TCNE]x magnetic films. Studies in this area should also lead to new classes of thin-film room-temperature magnets, e.g., V[Cr(CN)6]x electrochemically prepared.
Accordingly, the present invention includes spin valves (such as those shown in FIGS. 1 and 3), spin tunnel junctions, spin transistors and spin light-emitting devices that use the arrangements of the present inventions.